1. Field of the Invention
The present invention relates generally to the field of fracturing wells. More specifically, the present invention discloses a method and apparatus for creating multiple fractures in wells using propellants.
2. Statement of the Problem
Hydraulic fracturing has been used in the oil industry for many years and has undergone evolutionary changes throughout this period. It has worked effectively in stimulating oil production from wells that were drilled vertically where the borehole passes through hydrocarbon formations having a thickness on the order of tens of feet that can be effectively tapped by a single pattern of fractures extending radially outward from the borehole.
The advent of horizontal well drilling techniques allows a borehole to travel within a hydrocarbon bearing formation for up to thousands of feet. The borehole typically travels through a series of natural fractures that are at some angle with respect to the borehole. When hydraulic fracturing is attempted in such a horizontal borehole, a single fracture pattern usually occurs located along the weakest natural fracture in the formation. This result occurs because hydraulic fluid used in the fracturing process is supplied from the surface and cannot be pumped down the well quickly enough to overload the single fracture that has occurred. If the single fracture is not overloaded, subsequent fractures will not occur since the pressure in the borehole will not rise to the fracture extension pressure of the stronger fractures.
The need to cost-effectively recover oil from tight sand formations presents another challenge. Vertical wells drilled in tight sands are not easily completed using conventional hydraulic fracturing techniques. A significant portion of the fracturing fluid tends to leak off along the well into surrounding formation, rather than serving to fracture the desired hydrocarbon-bearing formation.
A number of devices and processes have been invented in the past relating to fracturing wells, including the following:
______________________________________ Inventor U.S. Pat. No. Issue Date ______________________________________ Hill, et al. 4,633,951 Jan. 6, 1987 Hill, et al. 4,683,943. Aug. 4, 1987 Austin, et al. 4,974,675 Dec. 4, 1990 Jennings 4,711,302 Dec. 8, 1987 Wolcott 4,522,260 June 11, 1985 Wolcott 4,446,918 May 8, 1984 Ford, et al. 4,391,337 July 5, 1983 Hane, et al. 4,329,925 May 18, 1982 Godfrey, et al. 4,039,030 Aug. 2, 1977 Blauer, et al. 3,937,283 Feb. 10, 1976 Mohaupt 3,313,234 Apr. 11, 1967 Graham, et al. 3,170,517 Feb. 23, 1965 Marx 3,136,361 June 9, 1964 Riordan 3,101,115 Aug. 20, 1963 Bourne 3,064,733 Nov. 20, 1962 Hanes 3,002,559 Oct. 3, 1961 Scott 3,001,584 Sep. 26, 1961 Rachford 2,766,828 Oct. 16, 1956 ______________________________________
The closest prior art references are believed to be U.S. Pat. Nos. 4,633,951 and 4,683,943 of Hill, et al. These patents disclose a method and apparatus for fracturing in which the well casing is first filled with a fracturing fluid. A gas generating unit containing shaped charges for perforating the well casing, and a propellant is suspended in the fracturing liquid within the well casing. The fracturing fluid is pressurized from the surface to a predetermined threshold value. The gas generating unit then perforates the well casing and simultaneously ignites the propellant. The propellant forces the fracturing liquid through the perforations and fractures the surrounding formation.
The Rachford patent discloses a system for fracturing in which the well casing is first perforated. A body of propellant is suspended in the fracturing liquid within the well casing and then ignited. The propellant forces the fracturing liquid through the perforations and fractures the surrounding formation.
Mohaupt discloses another system for hydraulic fracturing in which the fracturing liquid is driven by a non-detonating propellant.
Ford, et al., discuss a fracturing apparatus using a high velocity jet to first perforate the well casing. A gas propellant charge carried by the apparatus is ignited to expand the perforation and fracture the surrounding formation. Column 1, lines 25-44 provides a brief synopsis of the prior art relating to propellant fracturing.
Austin, et al., discloses a method of fracturing horizontal wells. A perforating gun carrying explosive charges is used to perforate the well casing. Hydraulic fracturing is then applied.
The Wolcott patents use explosive charges to create rubblized zones connecting horizontal bore holes to increase permeability.
The Scott and Riordan patents discuss the use of propellant to generate a pulse-like pressure boost to supplement the available surface pump pressure in hydraulic fracturing. This is similar in a general sense to the method discussed in U.S. Pat. Nos. 4,633,951 and 4,683,943.
The Bourne patent is another method of hydraulic fracturing in which the well casing is first perforated with shaped explosive charges carried by a perforating gun.
Graham, et al. discloses a method of hydraulic fracturing in which the fracturing liquid is driven by high pressure gas pumped from the surface.
Godfrey, et al., disclose a system in which both a propellant and a high explosive charge are used for fracturing. The propellant is ignited first, followed by detonation of the high explosive. The propellant serves to maintain pressure caused by the high explosive over a longer period.
Hane, et al., disclose an apparatus for fracturing using multiple explosive charges. The remaining references are only of passing interest.
3. Solution to the Problem
None of the prior art references uncovered in the search disclose a method of fracturing using a propellant to rapidly generate a sufficiently large volume of combustion gases, without detonation, to overload the weakest fracture, and thereby create multiple fractures. In a horizontal well, the present method creates a series of plane fractures that are roughly parallel to each other along the length of the bore hole. In contrast, a vertical well will experience a fracture in the least principle stress plane, similar to those produced by conventional hydraulic fracturing, plus a second fracture in a plane perpendicular to the least principle stress plane. The rapid pressurization of the well bore resulting from the burning of the propellant causes the fractures to propagate at rapid extension velocities. These extension velocities are on the order of the sonic velocity of the propellant combustion gases.